This invention relates to drill bits for use in woodworking and in particular to Forstner type drill bits.
Forstner type drill bits are shaft mounted drills that have a cylindrical cutting head with a pair of radial cutters that are perpendicular to the axis of the drill bit. These radial cutters allow for the drilling of flat bottomed holes. Located between the radial cutters, on the axis of the drill, is a center brad that acts to provide stability to the drill when engaged in drilling a hole. As the center brad extends longitudinally beyond the radial cutters only a short distance, perhaps only 10 percent of the diameter of the bit, and has a similarly small base, again perhaps only 10 percent of the diameter of the bit, the impression of the center brad left at the bottom of a blind hole is only small and tends not to diminish the flat bottomed aspect of the hole.
Directly leading and extending longitudinally behind each radial cutter is a chip channel that allows waste wood from the drilling operation to pass from the radial cutters to the space behind the cylindrical cutting head and then out of the so bored hole. Each chip channel has a radial width equal to the width of each radial cutter and is bounded by a trailing surface that is an extension of the radial cutter. Further the trailing surface is angled such that the radial cutter leads all other parts of the trailing surface.
Trailing behind and spaced slightly apart from each radial cutter are a spur and a following rim. Since the spur and following rim are positioned at the outer most diameter of the drill bit, the spur works to sever wood fibers that are at the diameter of the drilled hole. Each spur and following rim are set slightly outwardly longitudinally from the radial cutters such that each spur severs wood fibers and so defines a clean boundary for the drilled hole before the radial cutters uplift the wood that occurs between the spur and the center brad. The spur acts to sever wood fibers and the following rim acts as a stabilizer in the sense that it follows closely the circular groove cut in the work piece by the spur. This close following of the circular groove acts to keep the drill bit from wandering. As the inner surface of the following rim is generally conical and as the circular groove cut in the work piece is also made generally of a conical surface there is considerable tendency for the two conical surfaces to stay engaged and so there exists little tendency for the bit to wander. This engagement between the conical surfaces of the bit and work piece is sufficient to allow the bit to be used to drill a hole through the edge of a work piece wherein the center brad of the bit entirely overhangs the edge of the work piece being drilled. During this operation the sole source of stability of the bit, beyond the stability offered by the tool that is driving the bit, lies between the close engagement of the following rim surfaces and the circular groove in the work piece.
The outer surface of the following rim extends toward the shank of the bit and so forms a surface that offers additional stability to the drill bit in the drilled hole by being in close engagement with the bore of the so formed hole.
As there is a considerable amount of surface to surface contact between the drill bit and the work piece, particularly on larger sized holes, only a limited longitudinal displacement of the bit into the work piece is achieved per unit of longitudinal force applied to the bit. That is to say, the actual cutting rate into the work piece can be quite low. Further, due to the surface to surface contact, considerable heat is generated and time must be allowed between drilling operations for the drill bit to cool off as overheating of the drill bit will tend to alter the hardness and consequently shorten the life of the bit.
In an effort to improve the cutting rate of Forstner bits in general, a variation has been developed that has saw type teeth formed into the following rim. The saw teeth effectively replaces the conical inner surface of the following rim with a plurality of triangular saw tooth inner surfaces. The formation of saw teeth in the following rim of a Forstner drill can mean the reduction of more than 90 percent of the area of the conical inner surface that contacts the work piece during operation. By reducing this surface area, a greater displacement of the drill into the work piece is achieved per unit of longitudinal force applied to the drill thereby resulting in increased drilling rate and reduced friction and heat buildup experienced by the drill. However this loss of conical inner surface area can result in a considerable loss of aligning tendency that normally exists between the drill and the work piece thereby resulting in a reduced ability for the drill to form generally cylindrical holes in the work piece and reduced ability to form holes where the center brad of the drill overhangs an edge or is over a void in the work piece.
Additionally, the formation of saw teeth in the following rim of the Forstner drill bit also reduces the area of the outer surface of the following rim, but only by as little as 8 percent and as the outer surface of the drill is tapered slightly, having a smaller diameter as it progresses away from the following rim, a loss of area here does not greatly reduce the aligning tendency between the drill and the work piece.
In an additional effort to further improve the drilling performance and reduce both the resulting friction and the power requirements of saw tooth Forstner drills, flutes have been introduced. The flutes extend across the outer surface of the cylindrical cutting head from the following rim and generally downwardly away from the following rim. The flutes are angled slightly from the longitudinal direction in a forward direction such that the portion of the flute proximate to the following rim radially leads the remaining portion of the flute, further the width of the flute is approximately one half the pitch of the saw teeth of the drill. Two embodiments exist for the flutes, the first including flutes of generally parabolic shape having greatest width and depth at the following rim and extending to least width and depth at the shank edge of the cylindrical cutting head. The second embodiment includes flutes with generally parallel sides such that the flutes have constant width and depth over their entire extent. In either the parabolic or parallel sided flute case, the intersection of the flutes with the saw teeth of the drill are generally the same.
The advantage of the application of flutes to a Forstner drill that already includes saw teeth is that there is reduced friction between the drill and the formed bore by reducing the area of the outer surface of the drill that contacts the formed bore in the work piece. As saw tooth Forstner drills already have limited contact between the following rim of the drill and the work piece due to the portion of the following rim that were removed to form the saw teeth, the additional application of flutes to the outer surface of the drill does little to further reduce the area of contact between the inner conical surface of the following rim and the work piece. The disadvantage of the application of flutes to a saw tooth Forstner drill bit is that there is an increased tendency to wander. That is the drill is more likely to move sideways and thus create a hole that is not straight through the work piece.
It would be advantageous to provide a Forstner type drill bit that increases the cutting rate over a conventional Forstner drill bit but reduces the tendency to wander over the fluted saw tooth Forstner drill bit.
It has been determined that advantages can be realized by applying spaced apart depressions to the following rim of a prior art Forstner drill, where the pitch between depressions is greater than the extent of each depression. It has been further realized that such depressions can be achieved either by having the depressions extend generally perpendicularly to both the following rim and the axis of the bit and or by having the depressions extend generally perpendicularly to the following rim and parallel to the axis of the bit.
The present invention is a modified Forstner type drill bit which includes a shank and a drill head. The drill head includes a center brad, first and second radial cutters, first and second following rims and first and second chip channels. The center brad defines a longitudinal direction. The first and second of radial cutters extend radially from the center brad. The first and second following rims extend circumferentially from and following from in the direction of rotation the distal end of the first and second radial cutters respectively. Each following rim is divided into a plurality of segments, each have a top portion. The top portion of the segments of each following rim are generally in the same plane. The first and second chip channels are positioned adjacent to and before in the direction of rotation the first and second radial cutters respectively. The first and second chip channels are between the one of the first and second radial cutter and the second and first following rims. The shaft extends longitudinally from the radial cutters in the longitudinal direction and from the side opposed from the center brad. In one embodiment a plurality of depressions divide each following rim into segments. In another embodiment a plurality of elongate grooves divide each following rim into segments.
Further features of the invention will be described or will become apparent in the course of the following detailed description.